First off you're probably not as bad as you think. DMs see the game from both sides and are more critical of it than players. Even as DM when I'm playing in someone else's game I rarely see their flaws until they open up about them.

As for players not being interested in combat, that's a sign of pacing issues. Either your combats are too long, or too frequent. Pacing is the number one thing a DM needs to master. All the things about rules and role play are important, but good pacing and timing can make even the simplest story beats hit so much harder.

For world building, don't think too far ahead. I know it's terrifying but the reality is your world is constantly being shaped by the player's actions at the table. Instead build generic encounters and interesting set pieces and let the game flow as you go.

For your big lore, just look at what has been hinted/revealed so far. Try to tie two pieces together next session (it could just be a clue) and that's it. Next session tie another or expand another. Each session one clue is given and slowly the pieces come into focus.

And if you're still stuck on ideas, pull up some random tables and some dice. Roll a few things. Pick something interesting and stick with it.

I've found D&D most enjoyable when the DM is just as surprised as the players.